Projected beam smoke detectors are typically employed in warehouses, industrial facilities, and other locations having a very large area and/or comparatively high ceilings where a plurality of point-type detectors are unusable or otherwise impractical. Such devices are positively useful where dark-gray, black, and other smoke may be expected to be generated from consumption of material in the protected location, where the flow of conditioned air in the protected space is such that a rapid replacement of refreshed air can be expected, and in general where either large-volume protection or a low-level smoke detection capability is either desirable or important.
Projected beam smoke detectors typically employ a diverging beam of infrared energy that is projected from an infrared transmitter through a region to be protected and onto a spaced confronting infrared receiver. The intensity of the transmitted energy is attenuated in dependence upon the density and quality of smoke present along the optical path between the transmitter and the receiver. The receiver includes circuitry operative in response to the intensity of the received infrared energy to signal an alarm condition whenever it is out of prescribed bounds.
The receiver is usually mounted at the same height as and along the optical axis of the transmitter both to insure the reception of the transmitted energy and to prevent those false-alarms and failure-of-alarm situations that arise from misaligned optics. In the usual case, the receiver and the transmitter are installed to secure, torsion-free supports with the transmitting and receiving elements roughly in alignment, and thereafter the light emitting and light receiving elements themselves are vertically and/or horizontally so displaced as to bring them into precise co-axial alignment.
For some application-environments, appropriate pre-existing confronting supports such as spaced walls in the region to be protected are unavailable so that one or more costly transmitter and/or receiver mounting posts must be severally provided therefor. Moreover, as the supports naturally settle and/or are rotated by mechanical building stresses the transmitting and receiving elements mounted thereto tend to optically mis-align. If unnoticed, the undesirable possibility then arises of either a failure-of-protection situation or a false-alarm situation. Often the movement is of such a magnitude as to be beyond the range of compensation of the horizontal and vertical optical element adjustment capability, necessitating a further costly and time-consuming re-mounting and re-alignment procedure.
The transmitter and receiving heads are commonly employed in application-environments subject to undesirable electrical interferences that may give rise to failure and false alarm situations. One particularly troublesome failure and false alarm situations. One particularly troublesome interference is produced by flourescent lighting such as would be present in a warehouse to be protected. In such cases and in dependence on the sense of the interferring flourescent effects the receiver electronics are subject to degraded performance that could unduly delay its detection of a possible alarm event and thereby allow an undesirable increase in the degree of fire and/or smoke damage.
Projected beam smoke detectors are commonly installed in the protected region and calibrated while the region is being used in its normal everyday manner. In many applications such as for industrial facilities the calibration is performed relative to the changing ambient pollution levels generated in the working environment. If the projected-beam smoke detector is installed during uncharacteristically low-levels of pollution, it will then operate to produce unnecessary false alarms. If installed during uncharacteristically high-levels of work space pollution, it will operate to produce a failure-of-alarm situation. If the smoke detector is installed and calibrated at "nominal" working levels, the ambient characteristics of the work space environment still would vary in accordance with the type of activities being performed and thereby still give rise to the possibility of failure and false alarm situations.
After long periods of use in polluted environments, a film of dirt, dust, and grime builds-up on the transmitting and receiving elements even when mounted in well-sealed enclosures. The film provides an occlusion in the optical path that effectively acts to sensitize the detection capability of the beam smoke detector. In particularly polluted work spaces such as encountered in some manufacturing facilities, the degree of obscuration can be such as to repetitively produce an annoying false alarm signal indication so that a costly and burdensome periodic checking by maintenance personnel of the state of the optical elements is often employed to circumvent such a possibility.